This invention relates to laser driving methods and apparatus for driving a semiconductor laser used for example in reading out information signals from an optical disc.
Up to now, for reading out information signals from an optical disc, such as a phase change optical disc or from a magneto-optical disc, an optical pickup device, having a semiconductor laser as a light source, has been in use. In this optical pickup device, the semiconductor laser is driven by a laser driving device.
As for this laser driving device, it has been proposed to control the light emission output of the semiconductor laser using a so-called automatic power control (APC) circuit in which part of the light emitted by the semiconductor laser is received by a photodetector, such as a photodiode, and in which a driving signal for the semiconductor laser is controlled based on an output signal of the photodetector.
In this laser driving device, such a device has been proposed in which, for suppressing the so-called scoop noise produced by the semiconductor laser by the reflected light from the optical disc returning to the semiconductor laser, high frequency components are superposed by a voltage output type oscillator on the driving signals for the semiconductor laser.
Meanwhile, in the above-described laser driving device, since the superposed high frequency signal is of a high frequency and moreover the light waveform is of the complicated distorted wave, it has been difficult to control the amplitude. That is, since certain APC circuits cannot follow the high frequency signals superposed on the laser driving signals, amplitude fluctuations of the superposed high frequency signals cannot be suppressed, even though control of the light emission output of the semiconductor laser is possible as concerns the low frequency components, that is signals ranging from the DC signal to RF range signals. In this consideration, the conventional laser driving device uses a system in which the amplitude of the superposed high frequency signal is set to a desired value and supplied as a fixed output to the semiconductor laser.
However, the semiconductor laser is susceptible to variation due to temperature characteristics or chronological deterioration, such that, due to these factors, the semiconductor laser tends to be fluctuated in light output for a constant light input. That is, as for the temperature characteristics, the slope efficiency is lowered with rise in the environment temperature. On the other hand, if the superposed high frequency signal is driven with a voltage, temperature characteristics of the differential resistance of the semiconductor laser cannot be disregarded, this tendency becoming the lower the higher becomes the environment temperature. In this case, the high frequency current tends to be increased, such that the peak output is likely to be higher in association with currentxe2x80x94light output (I-L) characteristics. Since the peak output is increased due to such variations, the information signals, attempted to be read out from a rewritable optical disc, may be inadvertently erased in the worst case.
It is therefore an object of the present invention to provide a laser driving device and a laser driving method in which, when a high frequency signal is superposed on the driving signal for suppressing the scoop noise, the amplitude of the high frequency signal can be controlled reliably and satisfactorily without dependency on temperature characteristics or chronological changes in the semiconductor laser.
In one aspect, the present invention provides a laser driving device including a voltage controlled oscillator for superposing a high frequency signal on a driving signal for a semiconductor laser, a photodetector for receiving at least a portion of the light radiated from the semiconductor laser, amplitude information extraction means for extracting the amplitude information based on an output signal from the photodetector input through a narrow band-pass filter or a high-pass filter configured for extracting high frequency, and comparator means for comparing the amplitude information obtained by the amplitude information extraction means to a reference value to control the voltage controlled oscillator based on the results of comparison.
In another aspect, the present invention provides a laser driving method including superposing a high frequency signal on a driving signal for semiconductor laser, receiving at least a portion of the light radiated from the semiconductor laser by a photodetector, inputting an output signal from the photodetector to amplitude information extraction means through a narrow band-pass filter or a high-pass filter configured for extracting high frequency, causing the amplitude information extraction means to extract the amplitude information which is based on the output signal, comparing the amplitude information to a reference value, and controlling the superposition of the high frequency signal based on the results of comparison.
In the laser driving device and in the laser driving method according to the present invention, the amplitude information of the high frequency signal, obtained from the photodetector, is converted into a DC signal level in the direct vicinity of the photodetector, so that the information on the superposed amplitude can be transmitted to the next stage without the necessity of taking the wiring layout into account.
Moreover, since the amplitude information is converted into the DC voltage level, the working range of the one-cycle transfer gain can be lowered to afford a degree of freedom in the designing of phase allowance of the feedback system.
Moreover, if the high frequency signal is the distorted wave, there is raised no particular problem in the feedback control system because negative feedback is applied with the DC signal.
In the laser driving device and in the laser driving method according to the present invention, the amplitude of the high frequency signal superposed on the driving signals for the semiconductor laser can be controlled easily.
This enables control of the light peak output, which it has been impossible to control with the conventional APC circuit, such that the peak output can be maintained constant without dependency on changes in characteristics of the currentxe2x80x94light output (I-L characteristics) or chronological changes in the semiconductor laser.
In this manner, the amplitude of the superposed high frequency signal can be stabilized without dependency on the changes in the characteristics of the semiconductor laser to evade inadvertent data erasure in the rewritable optical disc caused by superposition.
That is, the present invention provides a laser driving device and a laser driving method whereby, in superposing the high frequency signal on the driving signal for scoop noise suppression, the amplitude can be controlled reliably satisfactorily without dependency on temperature characteristics or chronological changes of the semiconductor laser.